Inactivation of gram-positive bacteria

ABSTRACT

A method for inactivating medically important Gram-positive bacteria including Methicillin-resistant  Staphylococcus aureus  (MRSA), Coagulase-Negative  Staphylococcus  (CONS),  Streptococcus, Enterococcus  and  Clostridium  species, comprising exposure to visible light, and in particular light within the wavelength range 400-500 nm.

The present invention relates to a method for inactivating medicallyimportant Gram-positive bacteria including Staphylococcus aureus andmethicillin (multi)-resistant Staphylococcus aureus (MRSA),Coagulase-Negative Staphylococcus (CONS), Streptococcus, Enterococcusand Clostridium species.

BACKGROUND OF THE INVENTION

Methicillin-resistant Staphylococcus aureus (MRSA) is becoming anincreasingly problematic micro-organism, with infection rates rising andeffective methods of control becoming more and more limited. In additionto the resistance of MRSA to antibiotics, there is a significant problemdue to the availability of few effective sterilisation methods forenvironmental decontamination; for example in air and on contactsurfaces. Public and media interest in the transmission and control ofMRSA is escalating and it is becoming one of the most significantproblems within the healthcare industry. Hospitals and nursing homes arethe worst affected areas. Furthermore, community-acquired MRSA is alsonow being recognised as an increasing problem, with transmissionoccurring in public and social areas such as public gyms and sportscentres.

As well as MRSA, other Gram-positive bacteria are known to cause healthproblems, particularly in the hospital environment. For example,Staphylococcus epidermidis, which is a Coagulase-Negative Staphylococcus(CONS), can cause infection, particularly in infants and in hospitalisedpatients who have received prosthetic implant surgery. Streptococcuspyogenes is a Gram-positive coccus commonly associated with infectionssuch as pharyngitis, pyoderma, scarlet fever, erysipelas, cellulitis,streptococcal toxic-shock syndrome, rheumatic fever, glomerulonephritis,bacteraemia and necrotizing fasciitis, often referred to as“flesh-eating bacteria”. Enterococcus faecalis (another Gram-positivecoccus) is a common cause of urinary tract and wound infections, as wellas other infections including bacteraemia, endocarditis and meningitisin severely ill hospitalised patients. Multi-antibiotic resistance isalso becoming a well-documented problem with enterococcal infections.Clostridium species, in particular C. difficile, have been associatedwith high mortality in elderly patients due to diarrohea-associateddehydration, medically known as antibiotic-associated pseudomembranouscolitis.

Many techniques have been proposed for destroying harmful bacteria, suchas MRSA. For example, U.S. Pat. No. 6,251,127 describes a photodynamicprocess for the inactivation of bacteria and fungal wound infectionsusing methylene blue or toluidene blue. Light energy in combination withphotosensitising agents is used to treat or detect pathologies of livingtissue, including cancer and microbiological pathogens. The light usedhas wavelengths ranging from about 450 nm to about 850 nm. Testsdemonstrate the efficacy of the light treatment in combination with thephotosensitising agents for the destruction of Staphylococcus aureus inin-vivo infected wounds; and for in-vitro destruction ofantibiotic-resistant Staphylococcus, Streptococcus, Enterococcus, E.coli, Pseudomonas, Haemophilus influenza and Candida albicans. Inaddition, wavelength spectra of activation of methylene blue andtoluidene blue in the presence of various concentrations of the abovebacteria and Candida have been provided.

Whilst in some environments, the methodology of U.S. Pat. No. 6,251,127may be useful, it nevertheless suffers from the significant practicaldisadvantage that photosensitising agents must be applied to thebacteria that are to be inactivated. A similar problem arises withUS2005/0049228, which also requires the combined use of aphotosensitiser and light; in this case, in the range of 500 nm to 580nm. The need for photosensitising agents is a significant limitation ofthese techniques.

An objective of the present invention is to provide a simple andeffective technique for inactivating selected bacteria, in particularMRSA, and more generally the Staphylococcus, Streptococcus, Enterococcusand Clostridium species.

SUMMARY OF THE INVENTION

A method for inactivating one or more pathogenic gram-positive bacterialcomprising exposure of the bacteria to visible light without using aphotosensitiser.

Preferably said bacteria are selected from Staphylococcus, in particularMRSA, CONS, Streptococcus, Enterococcus and Clostridium species.

It is understood that the term pathogenic is used in the context ofgram-positive bacterial species and/or strains, which are capable ofcausing disease or infection in a human or animal subject. It is alsounderstood that some bacteria are often commensal in that they are ableto colonise and/or live on/within a healthy host and not becomepathogenic unless or until the host becomes immunocompromised and/orunhealthy due to some other form of disease or injury, such as a wound.Such “potentially” pathogenic bacteria are encompassed by the inventionalso.

Moreover, the term inactivation is understood to mean that said bacteriaare killed, or damaged so as to reduce or inhibit bacterial replication.The methods and systems taught herein can therefore be considered asbactericidal and/or bacteriostatic and this may depend on thespecies/strain of bacteria, wavelength of light, dose, etc.

Exposing these bacteria to blue light, or white light containing bluelight, has been found to stimulate an inactivation process. An advantageof using light in the visible-wavelength region is that there is nodetrimental effect on human or animal health. Consequently, the methodcan be used for an extensive range of applications, such as airdisinfection, contact-surface and materials disinfection and, mostnoteworthy, wound protection and tissue disinfection.

According to another aspect of the invention, there is provided a methodfor inactivating pathogenic gram positive bacteria including at leastone of Methicillin-resistant Staphylococcus aureus (MRSA),Coagulase-Negative Staphylococcus (CONS), Streptococcus, Enterococcusand Clostridium species comprising exposure of the bacteria to visiblelight having a wavelength in the range 400-500 nm. The visible light mayhave a wavelength in the range 400-450 nm. The light may have awavelength in the range 400-420 nm. The light may have a wavelength of405 nm.

According to yet another aspect of the invention, there is provided asystem for inactivating pathogenic Gram-positive bacteria includingMethicillin-resistant Staphylococcus aureus (MRSA), Coagulase-NegativeStaphylococcus (CONS), Streptococcus, Enterococcus and Clostridiumspecies, comprising the means for exposing them to visible light havinga wavelength in the range of 400-500 nm. The wavelength of the lightused is preferably in the range 400-500 nm. The wavelength may be in therange 400-450 nm, and more specifically in the range 400-420 nm, withoptimal inactivation at 405 nm.

According to still another aspect of the invention, there is provideduse of visible light having a wavelength in the range of 400-500 nm,especially 400-420 nm for inactivating pathogenic gram positive bacteriaincluding at least one of Methicillin-resistant Staphylococcus aureus(MRSA), Coagulase-Negative Staphylococcus (CONS), Streptococcus,Enterococcus and Clostridium species.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention will now be described by way ofexample only and with reference to the accompanying drawings, of which:

FIG. 1 shows the total emission spectrum of a Hamamatsu Xenon lamp;

FIG. 2 shows in greater detail the ultra-violet emission spectrum of theXenon lamp of FIG. 1;

FIG. 3 is a plot of bacterial count of a methicillin-resistant S. aureusstrain as a function of time of exposure to light of wavelength greaterthan 400 nm;

FIG. 4 is a plot of bacterial count of a second methicillin-resistant S.aureus strain as a function of time of exposure to light of wavelengthgreater than 400 nm;

FIG. 5 is a plot of bacterial count of S. aureus NCTC 4135 as a functionof time of exposure to light of wavelength greater than 400 nm;

FIG. 6 is a plot of bacterial count of S. epidermidis NCTC 7944 as afunction of time of exposure to light of wavelength greater than 400 nm;

FIG. 7 is a plot of bacterial count of Streptococcus pyogenes NCTC 8198as a function of time of exposure to light of wavelength greater than400 nm;

FIG. 8 is a plot of bacterial count of Enterococcus faecalis as afunction of time of exposure to light of wavelength greater than 400 nm;

FIG. 9 is plots of bacterial count in a suspension of S. aureus NCTC4135 as a function of time of exposure to light for different wavelengthranges;

FIG. 10 is a plot of bacterial log reduction as a function of wavelength(400-500 nm) for S. aureus NCTC 4135;

FIG. 11 is plots of bacterial count in a suspension of S. aureus NCTC4135 as a function of time of exposure to light of wavelength greaterthan 400 nm for different light intensities;

FIG. 12 is a visual indication of the surface inactivation of S. aureusNCTC 4135 through exposure to light of wavelengths greater than 400 nm.Surface inactivation is evidenced by inhibition of S. aureus growth onthe areas exposed to this light;

FIG. 13 is a plot of bacterial count of S. aureus NCTC 4135 as afunction of time of exposure to light of 405 nm;

FIG. 14 is a plot of bacterial count of a methicillin-resistant S.aureus strain as a function of time of exposure to light of 405 nm;

FIG. 15 is a plot of bacterial count of Streptococcus pyogenes NCTC 8198as a function of time of exposure to light of 405 nm, and

FIG. 16 is a plot of bacterial count of Clostridium perfringens 13124 asa function of time of exposure to light of 405 nm.

DETAILED DESCRIPTION OF THE DRAWINGS

Exposing MRSA to blue light has been found to cause significantinactivation. This narrow range of wavelength is part of the white-lightspectrum. For all white-light sources, only a small fraction of thelight output is in this range, typically one or two percent. Hence, toprovide a sufficient amount of light and demonstrate the effectivenessof this technique, the source used was a Xenon lamp (Hamamatsu PhotonicsUK Limited). Emission spectra of the lamp are shown in FIGS. 1 and 2.The lamp was used in combination with an optical-fibre light guide and aselection of optical filters in order to allow exposure of theStaphylococcus aureus suspensions to specified wavelengths of visiblelight. The output of the light guide was maintained at a distance of 5cm from the sample during all experiments.

To demonstrate the effectiveness of the technique, various studies havebeen carried out. The bacteria used were as follows: Staphylococcusaureus NCTC 4135; methicillin-resistant Staphylococcus aureus LMG 15975;methicillin-resistant Staphylococcus aureus 16a (clinical isolate),Staphylococcus epidermidis NCTC 7944, Streptococcus pyogenes NCTC 8198Enterococcus faecali and Clostridium perfringens 13124. Each sample wasserially diluted to the appropriate concentration usingphosphate-buffered saline (PBS), plated out using nutrient agar (NA) andthen incubated at 37° C. for 24 hours.

Suspensions of methicillin-resistant Staphylococcus aureus LMG 15975 andclinical isolate 16a were prepared and exposed to visible light. Thelight was transmitted through a 400 nm long-wave pass filter (50%cut-off in transmission at 400 nm) before impacting on the bacterialsuspension. This allowed only wavelengths of 400 nm and above (visiblelight) to illuminate the sample. The results of these experiments areshown in FIGS. 3 and 4. From these, it can be seen that the lighttreatment causes significant reduction in the counts of both the culturecollection MRSA (LMG 15975) and the highly resistant clinical isolate(16a). The control data refer to samples that were untreated over thesame time interval.

Suspensions of Staphylococcus aureus NCTC 4135 were also exposed tovisible-light treatment. Again, the light beam was transmitted through a400 nm long-wave pass filter before impacting on the bacterialsuspension, allowing only the transmission of wavelengths of 400 nm andabove. From FIG. 5 it can be seen that the Xenon light source causedsignificant reduction in the Staphylococcus aureus count even with ahigh starting bacterial population of approximately 10⁷ colony-formingunits per millilitre (cfu/ml). Similar experiments were carried outusing Staphylococcus epidermidis NCTC 7944, Streptococcus pyogenes NCTC8198 and Enterococcus faecalis. The associated reductions in thebacterial population are shown in FIGS. 6, 7 and 8. In each of these asignificant reduction in the bacterial count is observed.

Exposure tests using a range of filters were carried out. Bacterialsuspensions were exposed to the following wavelength ranges for times upto 90 minutes: greater than 550 nm (using a 550 nm long-wave passfilter); greater than 500 nm (using a 500 nm long-wave pass filter),less than 500 nm (using a 500 nm short-wave pass filter); 400-500 nm(using a 400 nm long-wave pass filter and a 500 nm short-wave passfilter in combination); 450-500 nm (using a 450 nm long-wave pass filterand a 500 nm short-wave pass filter in combination); greater than 450 nm(using a 450 nm long-wave pass filter), and greater than 400 nm (using a400 nm long-wave pass filter). The resultant inactivation curves in FIG.9 allow only qualitative comparisons to be made since the filters do nothave sharp cut-off wavelengths and the light intensities falling on thesuspensions were different for the different curves. The results dohowever indicate that the wavelength region between 400 nm and 500 nmdoes provide a high rate of S. aureus inactivation.

Experiments were also carried out using bandpass filters each with a 10nm FWHM (full-width, half-maximum). Suspensions of methicillin-resistantS. aureus LMG 15975 (approximately 10⁵ cfu/ml population) were exposedto visible light transmitted through the following bandpass filters: 400nm, 405 nm, 410 nm, 415 nm, 420 nm, 430 nm, 440 nm, and 450 nm. Theintensity of the lamp was altered for each filter to ensure that thelight power at the suspension was the same for each measurement, thusallowing direct comparison of results. The results of these experimentsshowed that samples exposed using the 400 nm, 405 nm and 415 nm bandpassfilters have a reduced colony-forming-unit count/ml; that is, light ofwavelengths within these narrow bandwidths had an inactivating effect onthe S. aureus strains.

A more detailed analysis of wavelength sensitivity was performed usingsuspensions of S. aureus NCTC 4135, and this is shown in FIG. 10. Theresults show that samples exposed using the 400 nm, 405 nm, 410 nm, 415nm and 420 nm bandpass filters have a reduced colony-forming-unitcount/ml; that is, light of wavelengths within these narrow bandwidthshad an inactivating effect on the S. aureus strains. From these resultsit can be deduced that visible-light exposure over the wavelength range400-450 nm is the major inducing factor for Staphylococcal inactivation,with increased inactivation occurring over the range 400-420 nm andoptimum inactivation occurring at 405 nm. Moreover, it has been observedthat a lower dose is required at this wavelength and typically the doseis less than 200 J/cm², such as less than 100 J/cm².

In further experiments, Staphylococcus aureus NCTC 4135 suspensions wereexposed to different intensities of visible-light treatment. Thesemeasurements were made using the 400 nm long-wave pass filter, that is,for wavelengths greater than 400 nm. FIG. 11 shows the results of theseexperiments. It can be seen that as the intensity of the lightdecreases, so to does the inactivation rate. The specific doses requiredfor complete inactivation of Staphylococcal, Streptococcal andEnterococcal species using different filters and light intensities weredetermined Sample results are shown in the Table below:

WAVELENGTH DOSE J/cm²/log ORGANISM RANGE (J/cm²) reduction S. aureus4135  >400 nm (100% intensity) 630 126 S. aureus 4135 >400 nm (75%intensity) 729 145.8 S. aureus 4135 >400 nm (50% intensity) 648 144 S.aureus 4135 <500 nm 189.6 37.92 S. aureus 4135 400-500 nm   290.8 58.2MRSA 15975 >400 nm 1260 252 MRSA 16a >400 nm 945 189 S. epidermidis >400nm 840 168 NCTC 7944 Strep. pyogenes >400 nm 1440 288 NCTC 8198 E.faecalis >400 nm 2880 1440

The effect of visible-light exposure for surface decontamination wasalso examined. This was done by exposing S. aureus cells, which wereplated onto nutrient agar, to the light treatment (through a 400 nmlong-wave pass filter) prior to incubation. Examples of results areshown as the areas of growth inhibition on the culture plates in FIG.12.

A similar treatment system to that used with the Xenon lamp wasassembled using a 405 nm LED array as a light source. Experiments werecarried out using Staphylococcus aureus NCTC 4135, MRSA 16a,Streptococcus pyogenes NCTC 8198 and Clostridium perfringens 13124. Theassociated reductions in the bacterial population are shown in FIGS. 13,14, 15 and 16, respectively. The specific doses required for completeinactivation of Staphylococcus, Streptococcus and Clostridium speciesusing the 405 nm LED array were determined Sample results are shown inthe Table below:

DOSE J/cm²/log ORGANISM WAVELENGTH (J/cm²) reduction S. aureus 4135 405nm 36 7.2 MRSA 16a 405 nm 45 9 Streptococcus pyogenes 405 nm 54 10.8NCTC 8198 Clostridium perfringens 13124 405 nm 45 10.2

A comparison of the doses required for bacterial inactivation (5-logreduction) using light of wavelengths greater than 400 nm from the Xenonlamp and a 405 nm LED array is shown in the Table below:

DOSE (J/cm²) J/cm²/log reduction ORGANISM >400 nm 405 nm >400 nm 405 nmS. aureus NCTC 4135 630 36 126 7.2 MRSA 16a 945 45 189 9 Cl. perfringens13124 1440 54 288 10.8

The use of 400-500 nm, in particular 400-450 nm, wavelengths of visiblelight (blue light) has proved to be an effective means of inactivationof Staphylococcus strains, including MRSA, as well as CONS,Streptococcus, Enterococcus and Clostridium, with increased inhibitionrates in the 400-420 nm range and in particular, around 405 nm. Thisdemonstrates that a light source (continuous source, flashlamp, laseretc.) with output at wavelengths in these regions could potentially beused in clinical environments for the reduction in levels ofmethicillin-resistant Staphylococcus aureus, and other medicallyimportant Gram-positive species present in the air and on contactsurfaces and materials, and most importantly, could be used for woundprotection and tissue treatment. The exact parameters required woulddepend on the bacterial strain, the wavelength of the light being usedand the light intensity. These can be readily determined experimentally.

Variations of the disclosed arrangements are possible without departingfrom the invention. For example, although both a Xenon lamp with avariety of different filters and a 405 nm LED array have been used asthe inactivation source, it will be appreciated that any suitable lightsource can be used. Equally, although a particular experimentalarrangement has been described here, it will be readily apparent thatthe light source used could be included in, for example, a hand-helddevice or could be designed to operate in or around areas that have tobe kept free of MRSA. Accordingly the above description of the specificembodiment is made by way of example only and not for the purposes oflimitation. It is clear that minor modifications may be made withoutsignificant changes to the operation described.

1. A method for inactivating one or more pathogenic gram-positivebacteria comprising exposure of the bacteria to visible light withoutusing a photosensitiser.
 2. The method according to claim 1 wherein thebacteria are selected from the group consisting of Methicillin-resistantStaphylococcus aureus (MRSA), Coagulase-Negative Staphylococcus (CONS),Streptococcus, Enterococcus and Clostridium species.
 3. A method asclaimed in claim 1 or 2 wherein the visible light has a wavelength inthe range 400-500 nm.
 4. A method as claimed in claim 3 wherein thevisible light has a wavelength in the range 400-450 nm.
 5. A method asclaimed in claim 4 wherein the light has a wavelength in the range400-420 nm.
 6. A method as claimed in claim 5 wherein the light is inthe region of wavelength 405 nm.
 7. A method for inactivating one ormore pathogenic gram-positive bacteria comprising exposure of thebacteria to visible light having a wavelength in the range of 400-500nm.
 8. The method according to claim 7 wherein the bacteria are selectedfrom the group consisting of Methicillin-resistant Staphylococcus aureus(MRSA), Coagulase-Negative Staphylococcus (CONS), Streptococcus,Enterococcus and Clostridium species.
 9. A method as claimed in claim 7or 8 wherein the visible light has a wavelength in the range 400-450 nm.10. A method as claimed in claim 9 wherein the light has a wavelength inthe range 400-420 nm.
 11. A method as claimed in claim 10 wherein thelight is in the region of wavelength 405 nm.
 12. A system forinactivating one ore more pathogenic gram-positive bacteria comprisingmeans for exposing said bacteria to visible light having a wavelength inthe range of 400-500 nm.
 13. The method according to claim 12 whereinthe bacteria are selected from the group consisting of Gram-positivebacteria, including Methicillin-resistant Staphylococcus aureus (MRSA),Coagulase-Negative Staphylococcus (CONS), Streptococcus, Enterococcusand Clostridium species.
 14. Use of visible light having a wavelength inthe range of 400-500 nm for inactivating one or more pathogenic grampositive bacteria.
 15. The method according to claim 14 wherein thebacteria are selected from the group consisting of Methicillin-resistantStaphylococcus aureus (MRSA), Coagulase-Negative Staphylococcus (CONS),Streptococcus, Enterococcus and Clostridium species.